1. Field of the Invention
The present invention relates to an optical image detector and, more particularly, to an optical image detector and method for controlling illumination of the same, in which an optical image reflected from a subject by a light source is detected and the illumination of the light source and an electronic shutter are controlled according to the detected optical image, thereby allowing enhancing a quality of the optical image and optimizing power consumption.
2. Description of the Related Art
Generally, an optical image detector such as an optical mouse is provided with a light source having a predetermined quantity of light, and is an apparatus in which light of the light source is incident onto a subject and an image of the subject is detected through the incident light reflected from the subject.
Particularly, in the case of the optical mouse as an apparatus receiving light reflected from a bottom surface at all times, there is the light source intentionally irradiated at all times. Then, a current consumed by the light source is responsible for most of the power consumption.
This type of optical image detector for receiving the reflected light to detect the image of the subject, as shown in FIG. 1, includes a light source 10, an image sensor 12, an electronic shutter 13, an A/D converter 14, an image processor 15 and a system controller 16.
The light source 10 emits a constant illumination of light, and the image sensor 12 receives light reflected from the subject to output a photo-voltage (or photo-current) corresponding to a quantity of the received light.
The electronic shutter 13 responds to a shutter control signal shutter-ctrl to receive and accumulate the photo-voltage (or photo-current) which is outputted from the image sensor 12 for a shutter-on time, and then extracts optical signals from the accumulated photo-voltage (or photo-current).
The A/D converter 14 converts the optical signals extracted through the electronic shutter 13 into digital signals.
The image processor 15 performs image processing with the use of the digital signals transmitted from the A/D converter 14. Further, the image processor 15 responds to the digital signals to generate the shutter control signal shutter-ctrl for turning the shutter on or off and then transmit the generated shutter control signal shutter-ctrl to the electronic shutter.
The system controller 16 receives image signals outputted from the image processor 15, and drives the system to output the received image signals.
Recently, with a great enhancement of integration in semiconductor devices, the image processor 15 may simultaneously function as the system controller 16.
As for the conventional optical image detector configured and operated in this manner, when the illumination of the light source 10 is sufficient or when a light reflectivity of the subject 11 is high, the reflected light may be incident onto the image sensor 12 in sufficient quantity. Hence, although the electronic shutter is exposed for a short time, a ratio of an optical signal to an optical image noise inputted from the image sensor 12 is high and the optical image can be given in stable quality.
On the contrary, when the illumination of the light source 10 is not sufficient or when the light reflectivity of the subject 11 is low, the reflected light may not be incident onto the image sensor 12 in sufficient quantity. Thus, the shutter-on time of the electronic shutter 13 is to be increased so as for the image sensor 12 to receive the sufficient quantity of the reflected light to obtain an accurate optical image of the subject 11.
FIG. 2 shows an integrated circuit in which the image sensor 12 and the electronic shutter 13 of FIG. 1 are connected.
As shown, the circuit of FIG. 2 is composed of a photo diode PD for generating a photo-current corresponding to a quantity of incident light, a transistor Q1 for amplifying the photo-current generated by the photo diode PD, a capacitor C for generating voltage signals proportional to the photo-current amplified through the transistor Q1, a first switch SW1 for charging electric charges of the capacitor C in response to reset signals, and a second switch SW2 for controlling paths of the transistor Q1 in response to a shutter control signal shutter-ctrl.
An operation of FIG. 2 configured as above is as follows.
First, when reset signals are applied to the first switch SW1 in order to perform an initialization operation, the first switch SW1 is turned off, and driving power is applied to the capacitor C. As a result, the capacitor C charges a quantity of electric charge corresponding to the driving power.
When the initialization operation is completed, the photo diode PD generates the photo-current proportional to the quantity of incident light. The generated photo-current is amplified at a current amplification factor hfe of the transistor Q1.
In this state, when the shutter control signal shutter-ctrl is applied to the second switch SW2, the second switch SW2 is on, and the charged electric charges of the capacitor C are discharged in proportion to the amplified photo-current. Then optical signals proportional to the generated current are generated.
Subsequently, when a shutter-off period becomes and the shutter control signal shutter-ctrl is applied to the second switch SW2, the second switch SW2 is off, and the generated optical signals are buffered through a buffer 20 and outputted.
In this manner, when the total quantity of light incident onto the photo diode PD is not sufficient, the second switch SW2 functioning as the electronic shutter is turned on for a long time, so that an insufficient quantity of light is fully accumulated and converted into the optical signals. At this time, the optical signals include a intrinsic level of noise generated by noise sources of the photo diode PD and the transistor Q1.
As a result, when the quantity of light incident onto the image sensor is little, a difference between values of the noise and the illumination of the optical signals is decreased as shown in FIG. 3A.
Thus, when the shutter-on time is increased in order to compensate the illumination values of the optical signals, the illumination values of the optical signals are increased together with the noise level as shown in FIG. 3B.
When the optical signals of FIG. 3B, which has a relation between the values of the noise and the illumination, is shown in the state of the image, there is generated a problem in that the noise is mixed into the image as shown in FIG. 4A.
Further, when the shutter-on time of the electronic shutter is increased, an after-image effect generated when the shutter-on time is increased in a general film camera is obtained. When the image of the subject in motion is detected, there is another problem in that the after-image effect becomes more and more serious as shown in FIG. 4B.